1. Field of Invention
This invention generally relates to image processing.
2. Description of Related Art
Devices that create, capture and/or reproduce color images have a color gamut (“gamut”) associated with them. A gamut is the range of colors that the device is capable of capturing or reproducing. When, for example, a color image created or captured by an originating device with an associated gamut, (e.g., a graphics program, a digital camera, or a scanner) is input into another device (e.g., a printer) to be reproduced, the originating device's gamut must be mapped onto the reproducing device's gamut.
Not all devices have equally large gamuts. An image captured by, or created for, one device may contain colors not reproducible on some other device. The input image in such a case contains “out-of-gamut” colors. Because something must be output for every input image pixel, a substitute color is chosen for each color that is “out-of-gamut.” In other words, any colors that are capable of being captured or created by the originating device (inside that device's gamut), but incapable of being reproduced by the reproducing device (outside that device's gamut) must be mapped, or adjusted, to colors that are within the reproducing device's gamut. Unless the originating device's gamut is quite small, some colors must be changed to produce an image that is entirely within the reproducing device's gamut, e.g., the printer's gamut.
Thus, when a color image captured or created on one device is reproduced by another device, some information is invariably lost due to gamut mapping. If that reproduced image is subsequently captured by a third device (e.g., scanned) the lost color information due to the initial gamut mapping is not recoverable. Furthermore, if the captured reproduced image is reproduced on, for example, a printer with a larger gamut, or if the captured print-ready file is re-purposed for a larger-gamut printer, the larger-gamut printer will only reproduce the captured image with colors adjusted for the smaller gamut of the original reproduction device. This is because most gamut mapping algorithms are not invertible, and hence even an attempt to re-map the image by inverting the gamut mapping prior to re-mapping for printing on the new device will fail.
Prior techniques for encoding information lost in gamut-mapping work only for electronic images, and will not survive printing and scanning. For example, U.S. Pat. Nos. 6,282,311, 6,282,312, and 6,282,313 teach a method for retaining information that is lost when representing digital camera images in a smaller gamut, such as sRGB encoding. A digital camera captures scenes whose gamut is typically much larger than that of sRGB. A standard gamut mapping of the camera signals to sRGB can thus irreversibly distort colors outside the sRGB gamut. U.S. Pat. Nos. 6,282,311, 6,282,312, and 6,282,313 propose computing a residual image between the original and gamut-mapped image, compressing the residual image, and storing the compressed residual image as metadata within the gamut-mapped electronic image. Because the residual image is stored electronically, this residual image is unavailable once the gamut-mapped electronic image has been printed.